In mobile communication systems, the transmission link suffers from a number of impairments. Two such effects are thermal noise and multipath fading.
Multipath fading can result in Intersymbol Interference (ISI) at the receiver if the delay spread of the channel is larger than the modulation period (‘Digital Communications’, Proakis, 2nd Edition, McGraw-Hill). Hence, in a given propagation environment, ISI will become more of a problem as the transmission rate increases. For communication systems aimed at providing medium to high data rate services, the presence of ISI can severely limit the link throughput and degrade the quality of service experienced by the user.
Some digital communication systems also introduce ISI by design. This is the case in the E-GPRS system, where the modulation pulse shape used to improve the spectral efficiency of the transmitted signal generates ISI. For more information, see 3GPP TS 05.04 V8.4.0 (2001-11), Technical Specification 3rd Generation Partnership Project; Technical Specification Group GSM/EDGE Radio Access Network; Digital cellular telecommunications system (Phase 2+); Modulation.
Another source of performance degradation experienced by a user of a Time-Division, Multiple-Access (TDMA) cellular communication system is interference generated by other users in the system using the same carrier or adjacent carriers. These interference effects, referred to as co-channel interference and adjacent channel interference respectively, can greatly reduce the capacity of a cellular system.
All of the impairments described above make it difficult for a receiver to reliably recover information that a transmitted signal intended to convey, leading to the use in receivers of complex algorithms for demodulating received signals. The implementation complexity of such algorithms will have a significant impact on a digital receiver in terms of overall silicon die size, processor speed, power consumption and memory requirements. Hence, the use of an efficient receiver architecture offering good transmission link performance is of considerable importance.
In order to improve the reliability of a communication link, Forward Error Correction (FEC) coding can be embedded in a transmitted signal. An FEC coding operation introduces redundancy to a transmitted signal and this redundancy can then be used at a receiver to improve the accuracy of estimates of the transmitted data that are generated by the receiver. However, for FEC coding in a transmitted signal to be most beneficial, it is important that such a signal is demodulated by a receiver in a format which can be best interpreted by an FEC decoding process within the receiver. A number of such receivers have been proposed in the past. See, for example, ‘Optimal decoding of linear codes for minimizing symbol error rate’, L. Bahl, J. Cocke, F. Jelinek, J. Raviv, IEEE Trans. on Information Theory, Volume: 20, March 1974; ‘A Viterbi algorithm with soft-decision outputs and its applications’, J. Hagenauer, P. Hoeher, GLOBECOM'89, Dallas, November 1989; ‘On the equivalence between SOVA and Max-Log MAP decodings’, M. P. C. Fossorier, F. Burkert, S. Lin and J. Hagenauer, IEEE Communications Letters, vol. 2, no. 5, May 1998; ‘Soft information in concatenated codes’, B. Rislow, T. Maseng, O. Trandem, IEEE Trans. on Communications, Vol. 44, Issue 3, March 1996; and ‘TCM on frequency-selective fading channels: a comparison of soft-output probabilistic equalizers’, P. Hoeher, GLOBECOM '90., December 1990. However, the implementation complexity of such prior receiver architectures is usually high.